Maybe you could find a heater made for a semi's diesel fuel tank.
Artic Fox & Webb come to mind as two manufacturers of those. Some are heated by the engine's coolant but they might have 12V models also. Could be $$$ unless you can find something used or in a junk yard.

Eric
http://s1199.photobucket.com/user/sparky234/library/My%2057%20Chevy%20Stepside?sort=3&page=1
'57 Chevy 3100 (Little Red)
'81 RD350 LC - Fun, Fun, Fun
'83 GS1100E - Pocket Rocket